Prismatic reflecting structure



Oct. 1, 1940. c. D. RYDER FRISMATIC REFLECTING STRUCTURE y Filed April2l /fwf/vm@` CHARLES D. AYDEP Lei/24 111111:? 4

Patented Oct. l, 1,940 y UNITED STATES PBISMATIC REFLECTIN G STRUCTURECharles D. Ryder, Covington, Ky., assignor to The National CoiortypeCompany, Bellevue, Ky., a corporation of Kentucky Application April 21,1939, Serial No. 269,187

1 Claim. (Cl. 38e-78) This invention relates to reflecting devices andin particular to auto-collimating reflectors such as are used in highwaysafety signals or upon vehicles.

One'object of this invention is to provide an auto-collimating reflectorhaving a wider angular range of auto-collimation for rays incident fromone side of the direction of approach than from the other side. v l

Another object is to provide an auto-collimating reflector having arelatively narrow range of self-reflection on one side thereof and arelatively .wide angle of self-reflection upon the other side thereofrelatively to the direction of approach of the'vehicle carrying thelight source.

Another object is to provide a prismatic refleeting device consisting ofa plurality. of 90 degree prisms formed on the back surface Aof atransparent. plate, alternate surfaces of 'these 0 prisms being furtherdivided into 9o. degree prisms running at right angles to. the main.prisms whereby the device will have a greater self-reflecting range fromone side of the direction of approach than from the other side thereof,thereby adapting the device to the lighting conditions encountered whenthe device is placed at the side of a road and is illuminated by anapproaching vehicle.

Another object is to provide a prismatic reflector of the typepreviouslydescribed wherein the prisms are formed integral with thetransparent plate, and preferably of a synthetic material other thanglass, and preferably a synthetic resin orother plastic substance.

95 ln the drawing:

Figure l is a front elevation of a prismatic reflector according to oneembodiment of the invention.

Figure 2 is a horizontal section taken along the 40 une 2-2 in Figure 1.

Figure 3 is a section through one of the elemental prisms taken alongthe line 3-3 in Figure 2.

Figure 4 is a diagrammatic view similar to a portion of Figure 2, butshowing the path of typical rays within the structure.

Figure 5 is a view similar to Figure 4 but taken at right anglesthereto.

50 Figure 6 ,is a perspective view looking toward' the rearward side ofthe prismatic reliector shown in Figures 1 to 5, inclusive. Figure 7 isa diagrammatic view illustratin the operation of the device when used`as a highas way signal and illuminated by an approaching vehicle bymeans of the light projected from the headlights thereof. 1 Y

In general, the device of this invention con-` sists of a plate oftransparent material having small right-angled prisms formed on the backsurface thereof. Alternate 'surfaces of these prisms are further dividedinto right angled` prisms running vperpendicularly to the main prisms.The prismatic reflector of the invention operates upon the principle oftotal reflection, 10. in such a manner.` that light incident upon thedevice is reflected back to the source through a fairly wide range ofangles on one side kof the direction of approach of a vehicle, butthrough a relatively narrow angular range on the other 15 side thereof.

In the use of devices of this character, such as for highway signals orupon vehicles, the device is usually located at the side of the road,hence as the vehicle with its headlights approaches the 'gg device therays of light will sweep through a relatively wide angle to one side ofthe direction of approach, but willV sweep through but a very tiny angleon the other side thereof.. The device of the present invention is soconstructed g5 and arranged that the rays incident upon it will bereflected back to the source, that is, to the approaching vehicle, notonly when the vehicle is ata considerable distance, but also almost upto the time when the vehicle passes the device. This 30 constructioneliminates the defect in previous devices which reflected back the raysthrough a relatively narrow angle on4 each side ofthe direction `ofapproach. Thus in prior devices the provision of even a relativelynarrow angle of operability on the sideaway from the road `wasunnecessary and wasteful, whereas the narrow angle on the side towardthe road was inadequate in that the device ceased to reflect back \therays as the vehicle came Within a short distance of the device. Thedevice of the present invention therefore adapts itself perfectly to theactual conditions of illumination existing in actual practice. l

Referring to the drawing in detail, Figure 1 45 shows the deviceaccording to one embodiment of the invention as mounted upon a supportIII having a pillar II rising therefrom and terminating in a easing I2which receives the optical part of the device, generally designated I3.

The optical part I3 consists of a flange-like portion I4 held within theannular casing I 2 and serving to space the forward portion I5 `awayfrom the rear wall I6 of the casing I2.

The rearward surface of the forward portion I is formed with prisms I 'Ihaving plane surfaces I8 alternately arranged with prismatic surfacesI9, the latter in turn being formed with prismatic surfaces 2i]alternating with prismatic surfaces 2|. The tiny prisms I9 are thereforeat right angles or perpendicular to the main prisms II. In order tofacilitate total reflection within the prisms I'I and I9, the surfacesthereof are formed at right angles, thereby constituting right angleprisms.

The optical action of the device is shown by tracing the paths of raysincident at different angles, as shown in Figures 4 and 5. In the righthand prism Il (Figure 4) the ray 24 is incident upon the front surface25 of the device perpendicularly. The ray 26 passes through the frontportion I5 Without deviation until it strikes the surface I8 of theprism I1, from which surface it is reflected to the opposite surfacehaving the tiny prisms I 9, the right-angled surfaces of which by totalreflection cause the ray 24 to emerge parallel to its enteringdirection. In this case, when the incident rays are perpendicular to thefront surface 25 of the device, all of the reflection takes place withinthe critical angle for total reflection and the device functionsperfectly for all rays entering the structure perpendicular to its frontface I5. This condition, however, rarely exists in practice due to thechanging angles of the rays from vehicle headlights as the vehicleapproaches 'the device.

On the other hand, assuming the device to be placed upon the right handside of the road, relatively to an approaching vehicle, it is highlyunlikely that any rays will be incident upon the 'front surface 25 atany considerable angle to the right of -the device. However, assumingsuch a ray to approach at an angle of approximately 6 degrees from thenormal to the front face 25, this ray 26 is the widest angle ray whichwill give complete total reflection from that direction, assuming theindex of refraction to be about 1.52, whereupon the critical anglerelatively tothe refleeting surface of the prism (surface I8) will be inthe neighborhood of 41 or 42 degrees. In

' other words, if this critical angle is 41 degrees,

the ray of light inside the transparent material strikes the reflectingsurface I8 forty-one degrees from its normal or perpendicular. 'I'hismakes the ray 26 incline 4 degrees from the normal to the front surface25 and this would give an angle of approximately 6 degrees from thenormal to the front face for the entering ray. A greater angle than thisangle would permi-t some of the light to pass through the surface I8,hence total reflection would not be obtained. Unless there was a slightcurve in the road prior to reaching the location of the device, thecondition shown in the middle of Figure 4 would not ordinarily exist.

As shown in Figure '7, however, when a vehicle 21 approaches the deviceI3 mounted at the right hand side of the road, the rays 28 given o bythe headlights 29 sweep through a relatively wide angle to the left ofthe direction of approach, as indicated by Figure 7. The optical actionunder these conditions is shown by the behavior of the ray 30 in theleft hand prism of Figure 4. This ray 30 is shown as striking the frontsurface 25 at an angle of 40 degrees, whereupon the angle of the ray 30after entering the portion I5 would be inclined about 251/2 degrees tothe normal to the front face 25. The ray 30, under these conditions,strikes the prism surface I8 at a large angle with the normal orperpendicular to that surface, or Well beyond the critical angle. It isthus evident that the device has a large angular range to the left and avery small angular range to the right. Vertically the device has a rangeof about 37 degrees above and below the horizontal plane. If, however,the device should be erected upon the left hand side of the road, inthis case the prismatic reflecting elements must be reversed. This isdone merely by turning them 180 degrees so that the arrangement is thereverse of that shown in Figure 4. The device may be provided with anindicating element such as an arrow 3I and a word or message 32. Thusthe arrow 3| will indicate the direction and will poin-t toward theroad, whereas the word 32 will indicate the object or location which itis desired to point out or against which danger the operator of theapproaching vehicle is warned.

In summation, therefore, the present invention provides anauto-collimating prismatic reflector having a very Wide auto-collimatingrange to one side of the device and a relatively narrow range to theother side thereof, thus accommodating the device to .the actualconditions encountered along a highway. It will be evident that thedevice may be employed not only as a stationary sign along the highwaybut also as a Warning indicator upon the front or rear ends of vehiclesto notify the operator of an approaching vehicle of their presence incase the lights or the parked vehicle are not in operation. On the otherhand, the relatively narrow angle of operability on the side away fromthe road is adequate for all practical conditions because rays do notapproach the device through a greater angle than this. On the otherhand, the provision of the wide angle of auto-collimating effectivenesson the side of the device toward the road gives the device a muchgreater efficiency and causes it to be selfluminant in appearance almostup to the time the vehicle passes it.

It will be obvious that the device may be combined with colored elementsor made of colored material in whole or in part, so as 'to give acolored appearance. The device is also preferably made from syntheticplastic material in order to be substantially indestructible.

It will be understood that I desire to comprehend within my inventionsuch modifications as come Within the scope of the claim.

Having .thus fully described my invention, what I claim as new anddesire to Isecure by Letters Patent, is:

An auto-collimating reflector comprising a solid transparent refractingbody having a smooth fron-t face upon which rays from a source of lightare adapted to impinge, the rear surface of said reflector being formedof contiguous right angle major prisms, the adjacent corresponding sidesof the prisms being formed alternately of a multiplicity of contiguousminor right angle prisms and smooth plane surfaces, said minor prismshaving their longitudinal axes perpendicular to the longitudinal axes ofthe major prisms, said smooth plane surfaces being of substantial widththroughout their length and interposed between the sides formed of saidminor prisms and having the opposed ends of said minor prisms contiguouswith the smooth surfaces and separated by said smooth surfaces, saidreflector having a wide angular range of reflection for light raysincident upon the front face of the reflector to one side of the normaland a small angular range of reflection for light rays incident uponsaid front face to the other side of the normal.

CHARLES D. RYDER.

